In a hybrid powertrain, an electric motor may be used to absorb and/or supply torque to improve powertrain efficiency and fuel economy performance. The hybrid powertrain may also include an energy storage device, such as a battery, that can power the motor or store absorbed energy. In one example, the efficiency of a hybrid powertrain may be improved by engaging the lockup clutch of a torque converter earlier when the internal combustion engine has low load by using the electric motor to provide additional torque to meet driver demand. However, as the battery state of charge decreases, the electric motor may be unable to provide supplemental torque. Thus, lockup of the torque converter may occur later so that the internal combustion engine may provide torque output to meet driver demand. One approach that adjusts the lockup schedule of the torque converter based on battery state of charge in order to improve powertrain efficiency is described in U.S. Patent Application No. 2005/0003928.
The inventors herein have recognized that the above approach may have some issues. In particular, during some operating conditions, the lockup clutch may be unable to lock the torque converter. For example, early torque converter lockup may result at a condition when a considerable torque differential may exist across the torque converter greater than that of the capacity of the lockup clutch. Thus, early torque converter lockup may cause deteriorated driving conditions due to the rough or unsuccessful engagement of the lockup clutch.
The above issues may be addressed by, in one example, a hybrid vehicle propulsion system comprising an internal combustion engine coupled to a multiple fixed-ratio transmission, one or more electric motors powered at least by a battery, wherein at least one of the internal combustion engine and electric motors supply torque to the drive wheels, and a controller for adjusting the torque output of the hybrid propulsion system, the controller adjusting the transmission to shift at a first level during a first battery state of charge, and adjusting the transmission to shift at a second level that is higher than the first level in a second state where the battery state of charge is lower than the first battery state of charge.
Thus, efficiency and fuel economy of a hybrid powertrain may be improved while meeting driver demand by adjusting the transmission shifting based on battery state of charge, while reducing deteriorated driving conditions.
Further, in another example, the transmission shifting and the torque converter lockup state may be adjusted based on battery state of charge so that torque differential of the torque converter may be reduced and smoother engagement of the lockup clutch may be achieved during selected engine conditions. By using the electric motor to provide supplemental torque to meet driver demand, the impeller (or transmission input) torque can be reduced so that the capacity of the lockup clutch will be sufficient to smoothly lock the torque converter at the desired operating condition. In this way, hybrid powertrain efficiency and fuel efficiency may be improved while facilitating smooth distribution of torque from the hybrid powertrain torque sources to meet driver demand.
Further, in yet another example, the transmission shifting and the torque converter lockup state may be adjusted based on the distribution of engine power between multiple propulsion paths in order to improve overall hybrid powertrain efficiency. In certain operating conditions, a portion of the engine power may be distributed to one propulsion path to power the electric motor (downstream of the transmission) and/or charge the battery while the remaining portion of the engine power will be transferred through the transmission in another propulsion path to provide power to the drive wheels. In these conditions, the transmission shifting and torque converter lockup scheduling may be adjusted to compensate for the reduction in net input power to the transmission. In this way, hybrid powertrain efficiency and fuel efficient may be improved while meeting the driver demand.
In still another example, a control architecture for a hybrid propulsion system may be provided that considers the tractive effort capabilities of the respective hybrid powertrain torque sources for a selected operating condition and adjusts the transmission shifting and the torque converter lockup state in order to distribute power flow accordingly. In particular, the control architecture may adjust the transmission shifting and torque converter lockup state based on the tractive effort capabilities of the electric torque sources, including the battery state of charge. In this way, hybrid powertrain efficiency and fuel efficiency may be improved while meeting driver demand.